This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-97050,filed on Mar. 29, 2002; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a voltage controlled oscillator.
2. Related Art
The voltage controlled oscillator employing a thin film bulk acoustic wave resonator has been noticed in late years. The thin film bulk acoustic wave resonator is an element that comprises a lower electrode, a piezoelectric film and an upper electrode formed over a cavity formed on a substrate in order.
Such thin film bulk acoustic wave resonator generates resonance at a frequency whose half-wavelength is equal to the sum of the thickness of the lower electrode, the upper electrode, both of the above being contacting with an air layer, and the piezoelectric film. The thickness corresponds to approximately 0.5 xcexcm to 3 xcexcm for the frequency of 1 GHz to 5 GHz. In this manner, it is advantageous especially for the resonance of high frequency region higher than GHz level.
FIG. 27 shows change of impedance in the vicinity of the resonant frequency for such thin film bulk acoustic wave resonator.
As shown in FIG. 27, the impedance presents the maximum value at the anti-resonant frequency fA, and the minimum value at the resonant frequency fR.
FIG. 28 further shows change of the phase in the vicinity of the resonant frequency for the thin film bulk acoustic wave resonator.
As shown in FIG. 28, the phase delays by 90 degrees at a frequency equal to or higher than the anti-resonant frequency fA or equal to or lower than the resonant frequency fR, and leads by 90 degrees at a frequency between the anti-resonant frequency and the resonant frequency.
A resonance circuit can be formed with the aid of such impedance characteristics and phase characteristics.
The anti-resonant frequency fA and the resonant frequency fR have a relation to the effective electromechanical coupling coefficient k2eff as follows.
k2eff=(fA2xe2x88x92fR2)/fA2xe2x88x922(fAxe2x88x92fR)/fAxe2x80x83xe2x80x83equation (1)
Therefore, the difference between the resonant frequency fR and the anti-resonant frequency fA is determined by the electromechanical coupling coefficient of the ferroelectric film k2eff.
When the voltage controlled oscillator is formed by using such thin film bulk acoustic wave resonator, the thin film bulk acoustic wave resonator is generally used by combining in series or in parallel with a varicap element such as a varactor whose capacitance varies in accordance with voltage.
In the case where the capacitance component and the thin film bulk acoustic wave resonator are combined like the above, the oscillation frequency lies between the resonant frequency and the anti-resonant frequency of the thin film bulk acoustic wave resonator. As evidenced by the equation (1), the range of the oscillation frequency is limited effectively by the electromechanical coupling coefficient k2eff.
Besides, because Q value of the varactor is far smaller than Q value of the thin film bulk acoustic wave resonator, Q value of the voltage controlled oscillator decreases, so that there is a problem that phase noise becomes large.
Furthermore, combining in series or in parallel with a reactor element such as a coil is often put to practical use to widen the variable range of the frequency. However, Q value of the voltage controlled oscillator further decreases because Q value of the reactor element is extremely low, and consequently there is a problem that the phase noise gets increased furthermore. Moreover there is a problem that the reactor element has a large variation per hour and lacks stability.
The present invention is intended to provide a novel miniaturized voltage controlled oscillator which has high stability of frequency, excellent property to the phase noise, small variation per hour, and a wide frequency variable range in view of the problems mentioned above.
To achieve the object mentioned above, the present invention provides a voltage controlled oscillator comprising:
at least one thin film bulk acoustic wave resonator comprising;
a ferroelectric thin film equal to or smaller than 10 xcexcm in thickness of perovskite type single crystal whose direction of polarization is aligned to the direction of thickness thereof; and
a pair of thin film electrodes provided so as to interpose the ferroelectric thin film between them, and changing a resonant frequency by a voltage applied between the pair of thin film electrodes:
a control voltage circuit applying the voltage between the pair of thin film electrodes: and
an amplifier coupled with the thin film bulk acoustic wave resonator constituting an oscillation circuit in cooperation with the resonator.
Moreover, the invention provides a voltage controlled oscillator comprising a plurality of the thin film bulk acoustic wave resonators elastically coupled to each other.
Furthermore, the invention provides the voltage controlled oscillator which is a balanced circuit combining the plurality of thin film bulk acoustic wave resonator.
In addition, the present invention provides a voltage controlled oscillator comprising; a first oscillation loop comprising a first thin film bulk acoustic wave resonator and a first amplifier, a second oscillation loop comprising a second thin film bulk acoustic wave resonator and a second amplifier and being insulated electrically from the first oscillation loop, and means for coupling elastically the first thin film bulk acoustic wave resonator to the second thin film bulk acoustic wave resonator.
The elastic coupling is preferably a reverse (negative) phase coupling.
It is preferable that the plurality of the thin film bulk acoustic wave resonators are connected in series to construct a series circuit, and the amplifier is connected in parallel to the series circuit, for oscillating at a frequency in the vicinity of an anti resonant point of the plurality of thin film bulk acoustic wave resonator.
It is preferable that the voltage controlled oscillator oscillates at a frequency in the vicinity of a resonance point of the first and the second thin film bulk acoustic wave resonators.
Additionally, the present invention provides a voltage controlled oscillator comprising; a series circuit or a parallel circuit comprising the thin film bulk acoustic wave resonator and a capacitor of a predetermined capacitance connected on at least one electrode of the thin film bulk acoustic wave resonator, and the resonant frequency of the circuit being changed by a voltage applied between the pair of the electrodes of the thin film bulk acoustic wave resonator.
Furthermore, the voltage control circuit preferably generates a DC voltage.
Moreover, the ferroelectric thin film is preferably an epitaxial growth film of barium titanate or lead zirconate titanate as a principal component.
The present invention further provides a voltage controlled oscillator comprising:
a first thin film bulk acoustic wave resonator comprising;
a first ferroelectric thin film equal to or smaller than 10 xcexcm in thickness of perovskite type single crystal whose direction of polarization is aligned to the direction of thickness thereof; and
first and second thin film electrodes provided so as to interpose the first ferroelectric thin film between them:
a second thin film bulk acoustic wave resonator comprising;
a second ferroelectric thin film equal to or smaller than 10 xcexcm in thickness of perovskite type single crystal whose direction of polarization is aligned to the direction of thickness thereof; and
a third thin film electrode interposing the second ferroelectric thin film, in cooperation with the second electrode, and deposited on the first thin film bulk acoustic wave resonator in the direction of thickness thereof and sharing the second electrode as a common electrode:
a control voltage circuit applying a voltage between the electrodes: and
an amplifier coupled with the first and the second thin film bulk acoustic wave resonators to constitute an oscillation circuit.
In this case, thickness including the first and the second ferroelectric thin films and the first to the third electrodes is preferably one fourth of the wavelength of a fundamental oscillation frequency.
In addition, the present invention provides a voltage controlled oscillator comprising:
a substrate:
a first thin film bulk acoustic wave resonator provided on the substrate comprising;
a ferroelectric thin film equal to or smaller than 10 xcexcm in thickness of perovskite type single crystal whose direction of polarization is aligned to the direction of thickness thereof; and
a first and a second thin film electrodes interposing the ferroelectric thin film between them:
a second thin film bulk acoustic wave resonator comprising a third and a fourth thin film electrodes to interpose the ferroelectric thin film between them, provided adjacent to the first thin film bulk acoustic wave resonator on the substrate, and sharing the ferroelectric film with the first thin film bulk acoustic wave resonator:
a control voltage circuit applying a voltage between the thin film electrodes: and
an amplifier coupled with the first and the second thin film bulk acoustic wave resonators, constituting an oscillation circuit.
Furthermore the invention provides a voltage controlled oscillator comprising:
a plurality of voltage controlled oscillator comprising;
at least one thin film bulk acoustic wave resonator comprising;
a ferroelectric thin film equal to or smaller than 10 xcexcm in thickness of perovskite type single crystal whose direction of polarization is aligned to the direction of thickness thereof; and
a pair of thin film electrodes interposing the ferroelectric thin film between them, and changing a resonant frequency by a voltage applied between the pair of thin film electrodes;
a control voltage circuit applying the voltage between the pair of thin film electrodes; and
an amplifier connected to the thin film bulk acoustic wave resonator to constitute an oscillation circuit in cooperation with the resonator:
means for oscillating the plurality of the voltage controlled oscillators with different frequencies: and
a multiplier connected to the plurality of the voltage controlled oscillators to output a difference of output frequencies of the voltage controlled oscillators.
In this case, the voltage controlled oscillator preferably comprises a plurality of capacitors connected selectively to the thin film bulk acoustic wave resonator so as to construct a series or a parallel circuit therewith, wherein the resonant frequency is controlled by means of combining with at least one of the capacitors.
As a variation, the voltage controlled oscillator preferably comprises a plurality of thin film bulk acoustic wave resonators connected selectively to a predetermined capacitor to construct a series or a parallel circuit therewith, wherein the resonant frequency is controlled by means of combining with at least one of the thin film bulk acoustic wave resonators.
Besides the present invention provides a voltage controlled oscillator comprising:
a substrate:
a thin film bulk acoustic wave resonator provided on the substrate, comprising;
a ferroelectric thin film equal to or smaller than 10 xcexcm in thickness of perovskite type single crystal whose direction of polarization is aligned to the direction of thickness thereof; and
first and second thin film electrodes provided so as to interpose the ferroelectric thin film between them:
a switching element comprising thin film electrodes provided adjacent to the thin film bulk acoustic wave resonator on the substrate, and sharing the ferroelectric film with the thin film bulk acoustic wave resonator, so as to interpose the ferroelectric thin film between them, and the ferroelectric thin film to be an actuator:
a control voltage circuit applying a voltage between the first and the second electrodes: and
an amplifier connected to the first and the thin film bulk acoustic wave resonators so as to constitute an oscillation circuit.